The present invention relates to a method and apparatus for removing heavy hydrocarbons (i.e. aliphatic hydrocarbons having six or more carbon atoms in total and aromatic hydrocarbons—also referred to herein as C6+ hydrocarbons and aromatics, respectively) from a natural gas stream. In certain preferred embodiments, it concerns a method and apparatus for removing heavy hydrocarbons from and liquefying a natural gas stream. The natural gas stream may be a stream that is already lean in aliphatic hydrocarbons having from 3 to 5 carbon atoms in total (also referred to herein as C3-C5 hydrocarbons) and/or a stream that is already lean in aliphatic hydrocarbons having from 2 to 5 carbon atoms in total (also referred to herein as C2-C5 hydrocarbons).
It is important to remove heavy hydrocarbons from a natural gas stream prior to liquefying the natural gas stream, as otherwise the heavy hydrocarbons may freeze in the liquefied natural gas (LNG) stream. It is also known that the heavy hydrocarbon components contained in natural gas feed streams can be removed using temperature swing adsorption (TSA) or by using a scrub column.
As is well known in the art, a scrub column is a type of separation device for removing less volatile components from a feed stream to produce a gas stream depleted in said less volatile components. The feed stream is introduced (as a gaseous stream or as two-phase, gas-liquid stream) into the scrub column, where it is brought into contact with a liquid reflux stream. The reflux stream is introduced into the column at a location that is above the location at which the feed stream is introduced, so that the falling stream of liquid comes into countercurrent contact with the rising stream of vapor originating from the feed stream, thereby “scrubbing” said vapor stream (i.e. removing at least some of the less volatile components from the vapor stream). Typically, the scrub column contains one or more separation stages, positioned below the location at which the reflux stream is introduced and above the location at which the feed stream is introduced, and composed of trays, packing, or some other form of insert that acts to increase the amount and/or duration of contact between the rising vapor and falling reflux streams, thereby increasing mass transfer between the streams.
In the case of treatment of a natural gas stream, a scrub column can be effective in removing all the heavy hydrocarbon components from the stream, but it must be operated at pressures lower than the mixture's critical pressure in order to achieve gas-liquid phase separation. The operating pressure of the column is lower than the optimum natural gas liquefaction pressure, which leads to lower liquefaction process energy efficiency. Also, stable scrub column operation requires sufficient liquid (i.e. reflux) to vapor flow ratio in order to avoid column dryout. The reflux for the column is typically provided by condensing a portion of the gas stream from the top of the column, and if the natural gas feed is in particular too lean in C3-C5 hydrocarbons and/or C2-C5 hydrocarbons (i.e. the concentration of these components is too low) it becomes very energy inefficient to maintain the required liquid to vapor flow ratio inside the column. Therefore, if the natural gas feed is lean in C3-C5 hydrocarbons and/or C2-C5 hydrocarbons and contains relatively high concentrations of heavy hydrocarbons the conventional scrub column technology is energy inefficient.
As is well known in the art, TSA involves at least two steps. During a first step (typically referred to as the “adsorption step”) a gaseous feed stream is passed through one or more beds of adsorbent at a first temperature and for a first period of time, during which the adsorbent selectively adsorbs one or more components of the feed, thereby providing a gaseous steam depleted in the adsorbed components. At the end of said adsorption step (which will typically be when the adsorbent is approaching saturation) introduction of the feed stream into the beds in question is stopped. Then, in a subsequent step (typically referred to as a “desorption step” or “regeneration step”) the beds are regenerated by desorbing the adsorbed components from the bed(s) at a second, higher temperature, and for a second period of time, sufficient to desorb enough of the adsorbed components to allow the bed or beds in question to be used for another adsorption step. Typically, during the regeneration step another gas stream (referred to as a “regeneration gas”) is passed through the bed to aid desorption and the removal of the desorbed components. In some TSA processes (often referred to as temperature pressure swing adsorption, or TPSA, processes), the regeneration step is also carried out at a lower pressure than the pressure during the adsorption step. In most TSA processes it is also the case that two or more beds of adsorbent are used in parallel, with the timings of the adsorption steps being staggered between the beds so that at any point there is always at least one bed undergoing an adsorption step, thereby allowing continuous processing of a feed stream. Each adsorbent bed may contain a single type of adsorbent material, or may contain more than one type of adsorbent material, and where there is more than one bed different beds may contain different materials (in particular where there are two or more beds arranged in series). Suitable types of adsorbent material for selectively adsorbing heavy hydrocarbons are well known.
TSA can be used to effectively remove heavy hydrocarbons from a natural gas stream at the optimum pressure for subsequent liquefaction of the stream, allowing for high liquefaction process energy efficiency. However, if the concentrations of heavy hydrocarbons are too high then the TSA vessel size and the regeneration gas requirements become economically infeasible. Therefore, TSA is effective in removing heavy hydrocarbons in natural gas liquefaction processes only when the concentrations of the heavy hydrocarbons are relatively low. In addition, a further complication is that the TSA adsorbent beds used for hydrocarbon removal need to be regenerated at high temperatures (i.e. 450-600° F., 232-315° C.). At these high temperatures there is a risk of the adsorbed heavy hydrocarbons cracking and producing coke, which will deactivate the adsorbent and be detrimental to productivity.
Prior art in this field includes documents WO 2009/074737, WO 2007/018677, U.S. Pat. No. 3,841,058, and U.S. Pat. No. 5,486,227 (which describe processes in which adsorption systems are used); and U.S. Pat. No. 7,600,395, U.S. Pat. No. 5,325,673, WO 2006/061400, US 2006/0042312, and US 2005/0072186 (which describe processes in which scrub columns are employed).
Accordingly, there is a need in the art for improved methods and apparatus for removing heavy hydrocarbons from natural gas streams, in particular where the natural gas stream has a relatively high concentration of heavy hydrocarbons or where the exact composition of the natural gas stream is liable to vary and/or may otherwise be unknown such that there is a risk of said stream having (at least at times) a relatively high concentration of heavy hydrocarbons.